The present invention relates to a polishing agent, particularly, to a polishing agent used for polishing a surface of a semiconductor wafer such as a silicon wafer or used in a CMP (Chemical Mechanical Polishing) method for polishing a film formed on a surface of a semiconductor substrate and to a polishing method using the same.
It is known to the art to subject a silicon wafer sliced from a silicon ingot to a primary polishing step, a secondary polishing step, etc. while increasing the accuracy of the abrasive, finally reaching a final polishing step.
In the final polishing, it is required to remove substantially completely the irregularity and warping from the wafer surface. Also, in the manufacture of an ULSI in recent years, removal of metal contamination from the wafer (wafer purity) is of high importance with progress in miniaturization of the device. Under the circumstances, it is of high importance to develop a high purity polishing agent which permits polishing of a high precision and which does not impair the purity of the wafer.
U.S. Pat. No. 3,715,842 discloses a polishing agent consisting of an inorganic oxide such as silicon dioxide, a water-soluble cellulose derivative, and an alkali. It is taught that the presence of the water-soluble cellulose is effective for suppressing the polishing damage done to the wafer during the polishing treatment.
The polishing apparatus using the particular polishing agent comprises a polishing cloth stretched on the polishing surface, a polishing disc rotated by, for example, a motor, and a suction disc capable of rotatably supporting a substrate and pressing the rotating substrate against the polishing disc.
In the ordinary method of performing a polishing treatment by using the polishing apparatus, the surface of the rotating substrate which is to be polished is pressed against the polishing cloth on the rotating polishing disc, and a slurry-like polishing agent is applied to the polishing cloth so as to supply the polishing agent between the polishing cloth and the substrate. The polishing technology utilizing the particular polishing apparatus is employed in the manufacture of a semiconductor device, a liquid crystal display device, etc.
In general, a semiconductor device such as an IC or LSI is manufactured via various steps including a designing step for designing an integrated circuit of a semiconductor substrate, a mask preparation step for depicting electron beams used for formation of the integrated circuit, a wafer preparation step for preparing a wafer of a predetermined thickness from a single crystalline ingot, a wafer processing step for forming a semiconductor element such as an integrated circuit on the wafer, an assembling step for separating the wafer into individual semiconductor substrates and for packaging the separated semiconductor substrates to prepare a semiconductor device, and an inspecting step.
In the wafer processing step, an optional material such as a polycrystalline silicon film, a silicon oxide film (SiO2) or a silicon nitride film (Si3N4) is buried in a groove portion such as a trench or a contact hole, followed by flattening the surface of the buried material. An etch back RIE (Reactive Ion Etching) is known as the flattening method.
However, various problems remain unsolved in the etch back RIE method. For example, the etch back RIE method requires a large number of process steps such as a coating step of an etch back resist. Also, the wafer surface tends to bear a RIE damage, it is difficult to achieve a satisfactory flattening, the manufacturing apparatus is rendered complex because vacuum-based devices are used, and a dangerous etching gas is used.
Such being the situation, the CMP method is being studied in place of the etch back RIE method as a flattening method in the wafer processing step.
A high accuracy is required for the polishing agent used in the CMP method, which is for flattening a surface of a semiconductor device in manufacturing the semiconductor device. Particularly, where the polishing treatment is applied to a silicon wafer itself or a polycrystalline silicon film formed on the wafer, it is proposed to use a polishing agent containing a water-soluble polymer in order to suppress generation of water marks.
Since a clean silicon surface exhibits a water repellency, a silicon or polycrystalline silicon film repels water in the polishing step and the cleaning step so as to form a mass of dust called a water mark. The water mark lowers the cleanliness of the wafer and the yield of the device in some cases. To overcome the problem, a water-soluble polymer is added to the polishing agent so as to form a hydrophilic film on the wafer and, thus, to prevent generation of the water mark.
In general, the polishing agent used for polishing a surface of a substrate, e.g., a silicon wafer, and a film formed on the substrate surface comprises abrasive such as silica particles, water as a solvent, a water-soluble cellulose for forming a hydrophilic film and, as required, a pH controller such as ammonia.
The water-soluble cellulose generally available on the market contains a large amount of alkali metal impurities such as Na, with the result that the wafer is contaminated with Na so as to bring about short-circuiting of the wiring formed on the wafer. To overcome this difficulty, it was customary in the past to suppress the Na content of the polishing agent to about 5 to 10 ppm.
For polishing a substrate or the like, the polishing agent is diluted with a dispersant such as a pure water or an ionic solution. In polishing, for example, a semiconductor wafer, the polishing agent is diluted with the dispersant such that the diluted polishing agent is xc2xd to {fraction (1/10)} as thick as the original polishing agent. In polishing a film formed on the semiconductor substrate, the polishing agent is generally diluted such that the diluted polishing agent is about {fraction (1/20)} as thick as the original polishing agent.
The dispersant used for the dilution is applied through a dispersant nozzle mounted separately from a polishing agent nozzle for applying the polishing agent to the polishing cloth. It follows that the polishing agent is diluted with the dispersant on the polishing cloth.
It is also possible to use two nozzles for applying two kinds of the polishing agents differing from each other in composition to the polishing cloth so as to achieve a desired polishing.
However, some problems are inherent in the conventional polishing agent. In the polishing of, for example, a silicon wafer or a device, the wafer is contaminated with the polishing agent. Where the washing after the polishing is insufficient, the yield of the device tends to be lowered.
Also, even if the addition amount of an alkaline agent is held constant by adding a water-soluble cellulose having the Na content lowered to some extent, the properties of the polishing agent such as a pH value are not made constant, with the result that, where the polishing treatment is carried out repeatedly, the polishing stability, etc. are rendered unsatisfactory.
It should also be noted that an alkaline agent having a high vapor pressure such as ammonia is used in the conventional polishing agent. Naturally, the alkaline agent tends to be evaporated easily, leading to problems in the storing properties of the polishing agent and in the stability of the polishing agent during the storage. Therefore, efforts have been made to develop a high purity developing agent excellent in stability.
Further, used in the conventional polishing apparatus are, for example, silica particles as the abrasive, with the result that an over-polishing tends to take place. Therefore, where a polishing treatment is applied to a CVD oxide film buried in a groove of a semiconductor substrate, an undesired dish-like recess tends to be formed on the surface of the CVD oxide film within the groove. In addition to the dishing problem, a corner portion of the groove of the semiconductor substrate itself tends to be etched. In this case, polycrystalline silicon of n+-type or p+-type or a residual metal tends to remain in a recess in the subsequent process steps, giving rise to problems such as an abnormal resistance of the polycrystalline silicon and short-circuiting of the wiring. Therefore, it is necessary to diminish the amount of the abrasive as much as possible.
A first object of the present invention, which has been achieved in view of the above-noted problems inherent in the prior art, is to provide a polishing agent which is excellent in stability and preservation capability for each lot and which does not do damage to the substrate or film to be polished.
A second object of the present invention is to provide a method of polishing a semiconductor wafer, which does not do damage to the substrate or film to be polished, which does not bring about non-uniformity among lots, and which permits satisfactory final polishing.
Further, a third object of the present invention is to provide a method of polishing a semiconductor wafer, which permits flattening a film to be polished on a semiconductor substrate by a CMP method without bringing about a dishing problem and a non-uniformity among lots.
The polishing agent of the present invention comprises abrasive containing silica particles as the main component, water as a solvent, and a water-soluble cellulose, and is featured in that, where the polishing agent contains C % by weight of the water-soluble cellulose, an alkali metal impurity content of the polishing agent is 5C ppm or less.
The method of the present invention for polishing a semiconductor wafer comprises a final polishing step which is carried out by applying a polishing agent to a polishing surface of a semiconductor wafer sliced from an ingot and subjected to a polishing treatment several times, said polishing agent comprising abrasive containing silica particles as the main component, water as a solvent, and a water-soluble cellulose, an alkali metal impurity content of the polishing agent being 5C ppm or less where the polishing agent contains C % by weight of the water-soluble cellulose.
The other method of the present invention for polishing a semiconductor wafer comprises a chemical mechanical polishing step which is carried out by supplying a polishing agent onto a film formed on the semiconductor wafer, said polishing agent comprising abrasive containing silica particles as the main component, water as a solvent, and a water-soluble cellulose, an alkali metal impurity content of the polishing agent being 5C ppm or less where the polishing agent contains C % by weight of the water-soluble cellulose.
In the polishing agent of the present invention, the alkali metal content of the water-soluble cellulose is controlled at a low value, with the result that the purity of the wafer after the polishing treatment is not lowered.
It should also be noted that abrasive are used in the polishing agent of the present invention in combination with the water-soluble cellulose having the alkali metal content controlled at a low value. As a result, the polishing agent is free from non-uniformity among lots, exhibits excellent stability in properties, particularly, in pH value, and permits achieving a stable polishing free from non-uniformity.
The polishing agent of the present invention, which is adapted for use in a final polishing of a wafer such as a silicon semiconductor wafer, permits polishing the wafer without giving rise to water mark formation. Also, in the case of polishing a film formed on a wafer, the film can be polished without bringing about a dish-like recess on the polished film surface.